This invention relates to an overcurrent protective circuit for protecting electrostatic self-turn-off devices, such as a MOS-FET, a Bi-MOS, or an IGBT (Insulated Gate Bipolar Transistor) by limiting the gate voltage of these devices.
Being capable of switching high-frequency currents, the electrostatic self-turn-off devices have been used for the purpose of making the frequency of noise due to switching operations higher than audio frequencies The IGBT, above all, has been used for power inverters and converters because of its capability of switching large and high-frequency currents.
As shown in FIG. 1, in the electrostatic self-turn-off devices, the collector current I.sub.C is determined by the magnitude of the gate voltage V.sub.GE, and the collector current I.sub.C increases as the gate voltage V.sub.GE increases.
As shown by an equivalent circuit in FIG. 2, an electrostatic self-turn-off device includes a collector-gate capacitance C.sub.GC and a gate-emitter capacitance C.sub.GE owing to its basic structure. Therefore, as the voltage between the collector and the emitter increases by .DELTA.V.sub.CE due to an increase in the collector current I.sub.C, the voltage between the gate and the emitter increases by .DELTA.V.sub.GE. EQU .DELTA.V.sub.GE ={C.sub.CG /(C.sub.CG +C.sub.GE)}.times..DELTA.V.sub.CE
Since C.sub.CG &lt; C.sub.GE, EQU .DELTA.V.sub.GE .apprxeq.(C.sub.CG /C.sub.GE).times..DELTA.V.sub.CE( 1)
By this .DELTA.V.sub.GE , the collector current I.sub.C is further increased and the increase in overcurrent is accelerated. It is possible to set the driving power source voltage at a low level in consideration of the above-mentioned voltage increase .DELTA.V.sub.GE However, this is not desirable because the saturation voltage during normal operation, namely V.sub.CE (sat) increases, thereby significantly increasing the saturation loss of the device.
Accordingly, there is disclosed an arrangement in Japanese Patent Application Laid-Open No. 63-95722, in which a Zener diode is connected between the gate terminal and the emitter terminal of an IGBT, so that the maximum value of the gate-emitter voltage of the IGBT is limited to the Zener voltage value of the Zener diode.
In addition, Japanese Patent Application Laid-Open No. 63-95728, reveals an arrangement in which a diode is connected between the gate terminal of an IGBT and the positive terminal of the gate driving power source, so that the maximum value of the gate-emitter voltage of the IGBT is limited substantially to the voltage value of the driving power source.
That is, the conventional overcurrent protective circuits are so arranged that when an overcurrent is detected, the gate voltage of the device is maintained at the driving power source or at a voltage value a little higher than the driving power source.